This invention relates to the protection of nickel-base articles with aluminum-alloy surface protective layers and, more particularly, to the protection of the internal surfaces of gas turbine airfoils.
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against an airfoil section of the turbine blades and vanes, which turns the shaft and provides power to the compressor and fan. In a more complex version of the gas turbine engine, the compressor and a high pressure turbine are mounted on one shaft, and the fan and low pressure turbine are mounted on a separate shaft. In any event, the hot exhaust gases flow from the back of the engine, driving it and the aircraft forwardly.
The hotter the combustion gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the combustion gas temperature. The maximum temperature of the combustion gas is normally limited by the materials used to fabricate the turbine vanes and turbine blades of the turbine, upon which the hot combustion gases impinge. In current engines, the turbine vanes and blades are made of nickel-based superalloys, and can operate at temperatures of up to about 1900-2150xc2x0 F.
Many approaches have been used to increase the operating temperature limits of the airfoil portions of turbine blades and vanes to their current levels. For example, the composition and processing of the base materials themselves have been improved, and a variety of solidification techniques have been developed to take advantage of oriented grain structures and single-crystal structures.
Physical cooling techniques may also be used. In one technique, internal cooling passages are present in the interior of the turbine airfoil. Air is forced through the internal cooling passages and out openings at the external surface of the airfoil, removing heat from the interior of the airfoil and, in some cases, providing a boundary layer of cooler air at the surface of the airfoil.
In combination with the above approaches, coatings and protective surface layers are used to inhibit corrosion and oxidation of the external and internal surfaces of the turbine airfoils. For example, the surfaces of the internal cooling passages may be protected with a diffusion aluminide coating, which oxidizes to an aluminum oxide protective scale that inhibits further oxidation of the internal surfaces. A number of techniques for applying the aluminum coating that forms the internal diffusion aluminide protective layer are known, including chemical vapor deposition, vapor-phase aluminiding, and above-the-pack techniques.
In the studies leading to the present invention, the inventors have recognized that it would be desirable to co-deposit other elements with the aluminum to improve the oxidation and/or corrosion resistance of the protective layer. The available deposition approaches have the drawback that it is difficult to apply alloys, containing modifying elements in addition to the aluminum, to the internal surfaces with good control over the compositions of the alloys. Consequently, the available processes have been directed primarily toward applying simple diffusion aluminides rather than more-complex diffusion aluminide alloys that take advantage of the beneficial effects of alloying elements co-deposited with the aluminum.
There is therefore a need for an improved approach to the depositing of aluminum-containing protective layers on specific areas of surfaces, particularly the internal surfaces of articles such as gas turbine airfoils. The present invention fulfills this need, and further provides related advantages.
The present invention provides a method for protecting surfaces of articles such as gas turbine airfoils made of nickel-base superalloys. The approach permits the composition of a protective layer applied to protected surfaces to be established by varying the alloy content of a condensed-phase alloy, rather than relying on the difficult-to-control properties of a vapor phase. Consequently, complex diffusion aluminide protective layers may be applied to protected surfaces, taking advantage of the beneficial properties introduced by the alloying elements. The present approach is readily used to prepare both internal and external protected surfaces, although its greatest benefits are realized when internal surfaces are protected.
A method for protecting a surface of an article includes furnishing an article made of a nickel-base alloy. A donor alloy of aluminum and at least one other element is provided and applied onto the protected surface of the article. The step of applying includes the steps of contacting the donor alloy to the protected surface of the article, and simultaneously heating the article and the donor alloy to a coating temperature that is greater than about 0.7 of the absolute solidus temperature but is such that the donor alloy remains in the form of a condensed phase contacting the protected surface of the article. The coating temperature is preferably from about 1700xc2x0 F. to about 2100xc2x0 F. The donor alloy is thereafter partially or completely interdiffused into the protected surface of the article. The interdiffusion is continued for as long as necessary to achieve a desired degree of interdiffusion, but typically occurs over a period of from about 1 to about 10 hours at the coating temperature.
The article of the most current interest is a component of a gas turbine engine such as a gas turbine airfoil. While both internal and external surfaces may be protected, the internal protected surfaces are of the most interest because it is more difficult to protect them with coatings and protective layers due to the absence of a line-of-sight access to the internal protected surfaces from other types of coating sources.
The alloy of aluminum includes at least one other element, examples being chromium, zirconium, hafnium, yttrium, cerium, platinum, and palladium, and mixtures thereof. Some specific examples are aluminum plus chromium in an amount of less than about 30 percent by weight of the alloy; aluminum plus platinum in an amount of less than about 64 percent by weight of the alloy; aluminum plus palladium in an amount of less than about 60 percent by weight of the alloy; aluminum plus zirconium in an amount of less than about 50 percent by weight of the alloy; aluminum plus hafnium in an amount of less than about 69 percent by weight of the alloy; aluminum plus yttrium in an amount of less than about 60 percent by weight of the alloy; and aluminum plus cerium in an amount of less than about 40 percent by weight of the alloy. Melting-point depressants such as silicon may also be included in the alloy. In the case of the preferred silicon melting-point depressant, the silicon content of the alloy is preferably less than about 20 percent by weight of the alloy. A particular advantage of the present approach is that donor alloys of more than two elements are readily prepared and used, as distinct from vapor deposition techniques where there is great difficulty in controllably depositing two or more elements simultaneously.
The alloy may be transported to the surface of the article to be protected by any operable approach. Examples are slurries and foams.
The present approach differs from conventional aluminiding techniques used for internal surfaces, where the aluminum is transported over relatively long distances from a source to the surface via the vapor phase. The vapor source may be several inches away from the surface to be protected, while in the present case the condensed-phase donor alloy is in direct physical contact with the surface to be protected. The relative rates of vapor transport of different elements, such as the aluminum and the alloying element(s), is difficult to regulate to achieve consistent compositions. Instead, in the present approach the donor alloy source of the aluminum and the one or more alloying element(s) is in a condensed phase in direct physical contact with the surface to be protected, preferably in the form of a continuous or discontinuous layer of the donor alloy on the substrate surface. As used herein, a xe2x80x9ccondensed phasexe2x80x9d is a solid, liquid, or mixture of solid and liquid, but not a vapor. In most cases substantially all of the donor alloy, including both the aluminum and the alloying element(s), is diffused into the protected surface, so that the amount of all of the elements that are introduced from the external source is known with certainty. It is recognized that there may be some incidental vaporization and subsequent redeposition of the donor alloy because there is a vapor pressure of the elements at the coating temperature, but the dominant mode of movement of the elements of the donor alloy to the protected surface is solid-state and/or liquid-state diffusion in the donor alloy, and solid-state diffusion in the nickel-base alloy.
The coating temperature is in excess of about 0.7 of the absolute solidus temperature of the donor alloy, but it may not be so high that the donor alloy is no longer in the condensed phase. (The ratio of a temperature measured on an absolute scale to the solidus temperature of an alloy measured on an absolute scale is sometimes termed the xe2x80x9chomologous temperaturexe2x80x9d, and in this case the coating temperature is at a temperature in excess of a homologous temperature of about 0.7.) The coating temperature may be in excess of the solidus temperature of the donor alloy, so that at least some liquid phase is present. It is critical that the donor alloy remain primarily in the condensed phase (i.e., a solid, liquid, or mixture of liquid and solid) in contact with the protected surface being treated.